Aortic cannulas are used to return blood to the aorta while the heart is by-passed during heart surgery. These cannulas are purposely made with small diameters (typically six to eight millimeters, but even smaller for pediatric applications) to minimize the disruption to the aorta, which in many heart surgery patients have advanced complex atherosclerotic lesions with adherent blood thrombi. The flow velocities through these small diameter cannula must be very high in order to maintain a satisfactory blood flow rate of about five to seven liters per minute. In at least some styles of conventional aortic cannula now in use, this high velocity resulted in "jet" flow emanating from the distal end of the cannula, which acted as a nozzle. It is believed that the force of this narrow jet stream may dislodge atheromatous material and/or adherent thrombi from the walls of the aorta, causing embolisms. As surgical equipment and techniques improve, making heart surgery available to older and more seriously ill patients, thrombo-atheroembolisms affect an increasing number of patients due to the increasing extent of atherosclerosis with age.
The size of aortic cannula may be constrained by the constricted size of the aorta of the typical heart surgery patient. Moreover, the ability to diffuse flow is restricted by the fragility of the blood, which is easily damaged by the shear stresses associated with turbulence.
The aortic cannulas of the present invention are adapted to provide high volume flow at relatively lower flow velocities than the conventional aortic cannulas presently available, thereby reducing the jet flow and consequently reducing the incidence of thrombo-atheroembolisms. Generally aortic cannulas constructed according to the principles of this invention comprise a diffuser that blocks some or all of the flow through the distal end of the cannula, and a plurality of outlet openings in the sidewall of cannula adjacent the distal end to maintain flow volume.
According to a first embodiment of this invention, the distal end of the aortic cannula is substantially blocked with a cap, and there is a tapering, preferably conical, diffuser extending upstream inside the lumen of the cannula toward the proximal end of the cannula. There are outlet openings in the sidewall of the cannula that permit the blood deflected by the diffuser to flow out of the cannula.
According to a second embodiment of this invention, the distal end of the aortic cannula is partially blocked by a diffuser having helical splines. There are a plurality of openings in the sidewall of the cannula between the splines on the diffuser to permit blood to flow out of the cannula. Additional outlet openings can be provided in the sidewall of the cannula upstream of the diffuser to reduce back pressure and the flow velocity from the distal end of the cannula.
The outlet openings provide for increased flow, thereby reducing the flow velocity from the cannula. The openings allow the flow to quickly establish a stable, more uniform velocity flow. In the first embodiment, the diffuser diverts the flow out of the outlet openings, minimizing hemolysis or other damage to the blood. In the second embodiment, the diffuser reduces the flow through distal end of the cannula, preventing jetting by imparting angular momentum to the fluid, and diverts a portion of the flow through the outlet openings in the sidewall surrounding the diffuser. Thus, the aortic cannula of the present invention reduces the high velocity jetting that can occur with some conventional aortic cannulas, while maintaining flow rate and minimizing damage to the blood.
These and other features and advantages will be in part apparent and in part pointed out hereinafter.